Put the Evaluate around the entire first argument:
f[x_]:=2x^3;
ParametricPlot[Evaluate[{D[f[x],x],x^2}],{x,0,1}];
Bob Hanlon
>
> From: "J. K. Smith" <rob at pi-overe.com>
To: mathgroup at smc.vnet.net
> Date: 2005/02/21 Mon AM 03:44:47 EST
> To: mathgroup at smc.vnet.net
> Subject: [mg54495] [mg54479] problems plotting a derivative
>
> I've tried a lot of ways but nothing has worked so far.
> The following statements are my problem simplified:
>
> f[x_] := 2x^3;
> ParametricPlot[{D[f[x], x], x^2}, {x, 0, 1}]
>
> When I run this I get error messages griping about numbers not being
> valid variables but it doesn't end up plotting anything. In this example
> I should see a straight line.
>
> I've tried using Evaluate[ D[f[x],x] ] and lots of other schemes which
> seemed to work in the past but I can't get ParametricPlot to digest any
> of them. Something about the result of that derivative doesn't yield
> something that can be plotted. But I don't know what property it is.
>
> Would you kind Mathematica gurus please get me thru this one (too)?
>
> Thanks.
>
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>